#include<iostream>
#include<vector>
#include<queue>
#include<cstring>
#include<cmath>
#include<map>
#include<set>
#include<cstdio>
#include<algorithm>
#define debug(a) cout<<#a<<"="<<a<<endl;
using namespace std;
const int maxn=1e5+1000;
typedef long long LL;
inline LL read(){LL x=0,f=1;char ch=getchar();	while (!isdigit(ch)){if (ch=='-') f=-1;ch=getchar();}while (isdigit(ch)){x=x*10+ch-48;ch=getchar();}
return x*f;}
LL a[maxn],cnt[maxn];
int main(void)
{
  cin.tie(0);std::ios::sync_with_stdio(false);
  LL t;cin>>t;
  while(t--){
    LL n,k;cin>>n>>k;
    for(LL i=1;i<=n;i++) cin>>a[i],cnt[i]=0,cnt[0]=cnt[n+1]=0;
    for(LL i=1;i<n;i++){
        cnt[i]=(a[i+1]-a[i]);
    }
    LL ans=0;
    for(LL i=1;i<n;i++){
        if(cnt[i]<0){
            if(i+k>n) continue;
            cnt[i+k]-=abs(cnt[i]);
            ans+=abs(cnt[i]);
            cnt[i]=0;
        }
    }
    for(LL i=n-1;i>=1;i--){
        if(cnt[i]>0){
            if(i-k<0) continue;
            cnt[i-k]+=abs(cnt[i]);
            ans+=abs(cnt[i]);
            cnt[i]=0;
        }
    }
    bool flag=1;
    for(LL i=1;i<n;i++){
        if(cnt[i]!=0){
            cout<<"-1"<<"\n";
            flag=0;break;
        }
    }
    if(1==flag){
        cout<<ans<<"\n";
    }
  }
return 0;
}
